A negative electrode material of a lead-acid battery contains barium sulfate, lignin sulphonic acid and carbon such as carbon black in addition to spongy lead. It is known that cycle life performance is improved when a bisphenol condensation product is added instead of lignin sulphonic acid.
Japanese Patent No. 4400028 discloses a method for producing a barium sulfate dispersion in which primary particles of barium sulfate are dispersed. In Japanese Patent No. 4400028, water-soluble barium sulfide is produced by reducing barite (natural barium sulfate) with coke, and reacted with sulfuric acid to produce the barium sulfate dispersion. Barium sulfate in the dispersion is washed with water without being dried, and is added to a negative electrode material for a lead-acid battery. Since the barium sulfate produced by reacting barium sulfide with sulfuric acid is used in the state of being dispersed in water, secondary particles formed by aggregation of primary particles do not grow. Japanese Patent No. 4400028 discloses that when 1 part by mass of a bisphenol condensation product based on 100 parts by mass of barium sulfate is added to the barium sulfate dispersion, the life performance of a lead-acid battery is improved.
The inventor has confirmed that when a negative electrode material contains a bisphenol condensation product, softening of a positive electrode material is accelerated. For this mechanism, the inventor made the following assumption. A bisphenol condensation product is more easily dissolved in sulfuric acid as an electrolyte solution in comparison with lignin sulfonic acid (hereinafter, referred to as “lignin”). When the bisphenol condensation product is eluted from the negative electrode into the electrolyte solution and reaches the positive electrode, softening of the positive electrode material is accelerated.
The inventor studied fixation of the bisphenol condensation product to the negative electrode. The negative electrode material contains lipophilic carbon, and thus allows the bisphenol condensation product to easily adsorb thereto. However, since carbon is added for forming a conductive network in the negative electrode material, it is not preferred that the insulating bisphenol condensation product is adsorbed to the negative electrode material. Thus, the inventor studied fixation of the bisphenol condensation product in the negative electrode material rather than depending on carbon.